Mutual conductance meter



June 8, 1937. L. E. BARTON ET Al. 2,083,357

MUTUAL CONDUCTANCE METER Filed May 27, 1933 Patented June 8, 1937 PATENTGFFICE MUTUAL CONDUCTANCE METER Loy E. Barton, Collingswood, N. J., andCarl C. Chambers, Lansdowne, Pa., assignors to Radio Corporation ofAmerica,

Delaware a corporation of Application May 27, 1933, Serial No. 673,198

17 `Claims.

Our invention relates to measuring instruments and particularly to aninstrument for measuring the mutual conductance of vacuum tubes.

The mutual conductance meters now in general use are operated by directcurrent, making necessary either the use of batteries or expensivelters.

The vacuum tube test sets using raw alternating current give anarbitrary reading of plate current which indicates only roughly thecondition of the vacuum tube. A further difficulty with present testsets is that the meter required for reading the plate current isnecessarily an eX- pensive one.

An object of our invention is to provide an alternating-current operatedmeter which will give an accurate measure of the mutual conductance of avacuum tube.

A further object of our invention is to provide an accurate mutualconductance meter which is comparatively cheap to manufacture.

A still further object of our invention is 'to provide a mutualconductance meter having a dial which is easy to read.

In practicing our invention, we provide a. plurality of transformerwindings which supply to the tube under test, voltages having peakvalues equal to the rated direct-current voltages for the tube. Theoutput of this tube is impressed upon a tuned circuit to which iscoupled an indi- 30 eating device. The grid bias of the tube and thecurrent flow in the tuned circuit are controlled by a potentiometer towhich is fastened a dial rotatable with the potentiometer arm andreading directly in mutual conductance units.

Other features and advantages of our invention will appear from thefollowing description taken in connection with the accompanying drawingin which Figure 1 is a circuit diagram of one embodiment of ourinvention; and

dial and certain associated apparatus used with the circuit shown inFig. l. v n Referring to Fig. l, the vacuum tube I under L test is shownin 4the upper left hand corner of the diagram. rlhe vacuum tube `3 shownat the right. of the diagramvis a unit of avacuum tube volt meter andforms a permanent part of the mutual conductance meter. Tube 3 comprisesa cathode 5, a control grid 1, and a plate 9. The tube I shown undertest in Fig. 1 has a heater lament II, an indirectly heated cathode I3,a control grid l5, a screen grid I1, anda plate I9.

The proper voltages for the vacuum tubes I i5 and 3 are supplied by aplurality of secondary Fig. 2 is a perspective view of our improvedwindings of a transformer 2I which has a primary Winding 23 adapted tobe connected to a suitable alternating current power supply such as theusual 110 volt line.

The cathode 5 of the meter tube 3 is supplied with heater current bymeans of the middle sec- 1 ondary winding 25. Heater current is suppliedto the tube I under test by means of another secondary Winding 21 whichis connected to the heater :lament II through the conductors 29 and 3l.Proper taps may be provided on winding 21 to heat the cathode of tube 3,if preferred.

The plate potential is supplied to the vacuum tube I by means of thesecondary Winding 33 which has its right hand terminal connected to theplate I9 through a conductor 35. The left hand terminal of Winding 33 isconnected to the cathode I3 of tube I through a conductor 31, Itheprimary 39 of an audio frequency transformer 4.I (which couples theVoutput electrodes of tube l to the input electrodes of tube 3), and aconductor 43. The plate circuit justtraced has an impedance which islo'w compared with the plate impedance of .tube 'Il iii The secondarywinding 33 also supplies plate potential to the meter tube 3, the plate9 of which is connected to a point on the winding 33 through adirect-current meter 45 `and a conductor 41. n The proper screen gridpotential is supplied to the vacuum tube I by means of the secondaryWinding 49 which has its right hand terminal connected to the screengrid I1 through the conductor 5I. The left hand terminal of the wind-ving 49 is connected to the cathode I3 of tube I through conductors 53and 43.

The secondary winding 55 of the audio `fre-r quency transformer 4I isconnected to the input electrodes of the meter tube 3, the upperterminal of the secondary being connected to the grid 1 through aresistor 51 of comparatively high resistance. The lower terminal ofthesecondary 55 is connected to the cathode 5 of tube 3 through conductors.31 and 59 and the secondary winding 25.

I The secondary 55 of the audio transformer is tuned to the frequency,or approximately the frequency, of the power supply, (60 cycles in casea 60 cycle power supply is used) by means of a condenser 6I shuntedacross the secondary winding.

The proper grid potential is supplied to the control grid of the tube Iby means of the secondary winding 63. secondary Winding I(i3 isconnected to the control grid I5 through a .conductor 65. The right handThe left hand terminal of thev terminal of the'winding 63 is connectedto the 'I'he Ahalf sine-wave plate fcurrent will-,be trans- 1 cathode I3of tube I through a yhigh resistance unit 61, through a grid biasingresistor Re-which is one section of a potentiometer 69, and through theconductor 29 to the cathode I3.

'I'he potentiometer 69 comprises the'resistance unit R4 and a resistancelunit R5 which are conswitch 1I is closed.

Irwin be noted that afmeren isis connected f across the secondarywinding 83. This is to enable theV personl testing the tube I to'adjusta potentiometer 15 A.until the voltage supplied to the'secondaries oftransformer 2| has the correct value,such an adjustment-beingdesirablebecause vof variations in line voltages.'v VBy including thepotentiometer 15 Yacross onlyfa few turns of the primary winding 23, asmall value of r'e' sistance may be used so that the. voltage drop`through it will be small and will not vary greatlywith different loads.1 1 Y j During a testythe voltage applied to the grid ofthe'tube J| willhave one of two values. VIf the switch ,1| is open, the value willbethat'of the voltage across the secondary winding 63, since there isno'current flowing through the potentiometer v|19, and no voltage dropintheresistor unit R4.

When ,the switch 1| is closed, current flowsy through the potentiometer69 and the voltage drop in the resistor R4 is subtracted from thevoltagev across the secondary winding 63. Thus, for example, when thesecondary winding ,63 applies a negative potential toA thegrid I5, thevoltage l.lropinthe resistor R4 decreases this negative potential.4 r a`All of thesecondary 4windings of the supply transformer V2| are woundinthe same directionl Therefore, when positive potential isappliedrtoVthesplaters of the tubes I and3and to the screen gridV of tube I,anegative potential is applied to theV control Vgrid of tube I. In otherwords', the voltages applied to the control rgrid vI5 and tothe plate I9are 180 degrees: out ofljphase.

It will be apparent that when the power supply switch 11 is closed, apulsating current will flow inthe plate circuit of the tube I throughtheV primary winding of the audio frequency trans-v I'his currentV willconsist of alternate L former 4 I. half cycles of the 60 cyclecurrent(assurning a 60 cycle supply is used), since no current will flowinthe plate circuit 'while the plate is negative.

ferred to the secondary of the audio frequency transformer 4|, andsincethe'secondary is tuned to 60 cycles, only aV 60 cycle voltageYwillbeimpressed upon the input electrodes of the-tube 3.

Sincethe control grid I5 goes positive at the time the plate I9Vgoesnegative, it is desirable to include the resistor unit 61 havingeacomparatively high resistance value to limit thevflow of gridV current.The resistorrIiI also prevents a heavy flow of Ycurrent in case thecontrol grid I5V and cathode I3 become. shorted or Vconnectedtogether.

From-thefabove description it is apparent thatwhen thefrsupply-switch11is closed, a 6V0 cycle pulsatingcurrent flows through the plate meter45, the current being pulsating since an alternating voltage isimpressedon plate 9.v This current has .a magnitude dependent upon thercharacteristicsofthe vacuum tube I under test.` It will also .beapparentv thatwhenfthetest switchll is closed,t he voltage drop intheg'rid biasing resistor R4 causes a change in the Clllitilt @Owingthrough the plate meter as, this change aise being dependent uponv thecharacteristics ofthe tube 1 I'under test. Knowing the change in gridvoltage caused byV closing switch 1|,'and knowing the Y .since themutualconductance equals nected across the secondary winding 21 WherraAY YA113,V Y Gm-fra micromhos where AI@ is the change'in plate current inmicroamperes,.and AE.; is the change in grid voltage.l ,Whereaiternating-'currentis used, this ratio is taken'when the plate of thetube is positive.

The change in plate current is determined inV lterms ofthe resistance ofa potentiometer 19,

comprising .arms R1 and Rz, which is shunted ywinding 39, which opposesthe plate current ow of the tube I (assuming variable tap 8|`is not atthe extreme left-hand position). The circuit which inducesY thisopposing current may be traced from the variable tap `8| on theVpotenti- A ometer 19 through the resistance unit-R3, which hasa highresistance value, through the Vprimary winding 39, and' through theconductors 43l and 29 to the left hand' terminal of potentiometer 19.That the voltage drop-acrossthe potentiometer VY`section'Ri induces anVopposing current flow in the primary winding 39, may be s een from thefollowing considerations. Plate current from tube I flows through'theprimary winding 39 only when"` the plate |9-is positive. At this timethe right'hand'terrninalof the secondary winding'21 is positive so thatthe tap '8| of the potentiometer 19 is positiver with respect to theleft handV terminal of potentiometer 19. Therefore, current flow causedby the voltage'drop in R1 is from the tap 8| through-resistor R3, andfrom the lower terminal of the primary winding'39 towards the upperterminal of that winding. f At this time, the plate current flow of tubeIlsv from the cathode I3, through Ytheconductor43 to the upper termi-Vnal'of the primary winding 39,and through the winding 39 toward itslower terminal.

:The change in plate current caused by the change in grid voltage isdetermined by noting the reading of the plate current meter. 45 when thetest switch 1|'is open, then closingithe test switch 1| (which causes a'change in the meter reading'unless tap 8| happens to be set at the ycorrect position), and'then adjusting the tap18| -on potentiometer 19until the reading of meter '45 has the original value. YIf the'potentiometer 19 has'lbeen properlyrcalibrated, the position of the tap8| then 'Y gives thermutual conductance value directly. 'I'here'asonfor-this will be explained more Vfully l hereinafter.

Y "Since'the meter 45 is Va verysensitive one, some form of adjustmentis necessary to keep itsy pointer onthe-scale when tubes-having widelydifferentY plate current values are tested. 'This adjustment is providedby the potentiometer shunted acrossthe secondary winding 21; The

voltagedrop across one section of the potentiometer 83 is impressedacross the primary 39 of the audio transformer 4| by means of a circuitwhich may be traced from4 the variable tap 85, through'a under normal orrated voltage conditions.

mutual conductance values apply only when cerresistor '81, the primarywinding 39, and the conductor 43, to the left-hand end of potentiometer83.

It is obvious that potentiometer 83 causes a ow of current through theprimary winding 39 opposed to the plate current of the tube I in thesame manner as the flow of opposition current from potentiometer 69. Itfollows that by adjusting the tap 85, the pointer of the meter 45 may bebrought to the desired test position for the tube under test.

Tube manufacturers publish charts showing what the mutual conductance ofthe different types of vacuum tubes should be when operated These tainspecified voltages are on the tube elements, and in the charts thesevoltages are given as direct-current voltages.

In order to test the tubes under the conditions specified in thesecharts while using alternatingcurrent for testing, in ourabove-described circuit the voltages applied to the tube electrodes(except the heater, which is supplied rated RMS voltage) are given peakvalues equal to the directcurrent voltages specied in the charts. Thenthe mutual conductance reading obtained by our meter will beapproximately the same as the chart value if the tube is in goodcondition. In effect, our meter determines the mutual conductance of avacuum tube at the instant that the electrode voltages have their peakvalues.

It will now be shown that the potentiometer I9 may be calibratedlinearly in terms of mutual conductance so that the position of tap 8|with respect to one end of the potentiometer gives a direct indicationof the mutual conductance value. It will be assumed that the test switch1| is open so that the grid voltage of tube is the voltage acrosssecondary winding 63, and that the tap 85 has been adjusted to bring thepointer of meter 45 to the test position. (At this test position thetube 3 has been adjusted to give best operation as a tube-volt-meter--)The variable tap on. the potentiometer 82 has previously been adjustedto make the maximum plate current through tube 3 the proper Value. Thisadjustment is anI ordinary tube-volt-meter adjustment and will bepermanent until a characteristic of tube 3 has changed, or until tube 3has to be replaced by a new tube.

The test switch is now closed whereby the voltage drop acrosspotentiometer section R4 is put in series with the voltage of secondary53. This voltage drop in R4 represents the change in grid voltage orAEg.

The closing of test switch causes an increase in plate current of tube I(ignoring the eiTect of potentiometer 19), as will be indicated by adecrease in the reading of meter 45, since the voltage drop in resistorR4 opposes the negative biasing voltage of secondary E3. The reading ofmeter 45 will decrease because the secondary 55 is so phased withrespect to voltage applied to anode 9 that the voltage applied to grid'l is negative when anode 9 is positive.

The phasing of the input voltage and plate voltage of tube 3 is notcritical, although the adjustment just described is preferred for'maximum sensitivity. If desired, resistor 51 may be shunted by acondenser so that grid will always go negative and the reading of meter45 will decrease with increase of the plate current of tube I no matterwhat the phasing is.

Switch 1| having been closed, the tap 8| of potentiometer 'I9 is nextadjusted until` the pointer of the meter 45 has been brought back to thetest position. This means that the voltage drop in the potentiometersection R1 is causing a ow of current (a bucking current) in the primaryWinding 3S which exactly balances the 60 cycle component ofthe-increasein plate current caused by the change in grid voltage.

Viewed in another way, the energy supplied to the tuned secondarycircuit by the bucking Acurrent due to potentiometer 19 is equal V(andopposite in phase) to the energy supplied thereto by the increase inplate current of the tube under test. It is apparent that the energysupplied to the tuned secondary circuit by the tube under test is justone half the energy that would be supplied if both half cycles wereflowing in the plate circuit instead of alternate half cycles.Therefore, the peak value of the balancing or bucking current must bemultiplied by two in order to obtain the peak value of AID during thehalf cycle the plate is .positive.

Let the peak voltage across secondary winding 21 be represented by VThen Then Gm=KR1 where K is a constant for a particular design (8) It isapparent from the above calculations that ifv the potentiometer 'I9 hasa linear resistance, it may be calibrated linearly in mutual conductanceunits.

The structure and arrangement of the dial and calibrated potentiometer19 will be understood by referring to Fig. 2. The resistance element ofpotentiometer 19 isr arranged in circular form, and mounted on asupporting member 89 at the rear of the vertical panel 8| of thecomplete mutual conductance meter. The Variable tap or contact member 8|of the potentiometer 19 is fastened to a rotatable shaft 93 which eX-tends through the vertical panel 9|. A knob 95 9| for operating thepotentiometer tap 8|.

u 'A dial 91 is fastened tothe shaft 93 atthejrear c'f the panel 9| torotate with the potentiometer contact member .8| for indicating theValue of mutual conductance. n The zero markon the dial is set on theshaft at the same point as the arm J8|. Near the outer edge, the,V dial91 is marked in mutual Vconductanceunits reading from to .Y 3800. `Ashield 99, which isset intoand sup- Y portedby the-panel 9|, has apointer orother Q,%P1 equals the percentof the resistance ofpotentiometer 19connected across the bucking" circuit. From aninspection of Fig; 2 it isappar- 30 ent 'that' %P1 also equals thepercent of thev dial scale that' is used. Therefore, 'the maximum lmutual conductance value that a meter will' read is found by letting %P1equall.

rIfheri when the person testing thevacuum tubes knows what value ofmutual conductance the tubes 40 should haveA for satisfactory operation.In practice, however, 'the vacuum tubes often will be tested byunskilled operators.Vl For this reason,

the dial markings and shield structure vdescribedv below are'desi-rable.

.15 Between the mutual conductance markings and the center of the dialthere are a plurality of concentric rings, each ring being divided intosegments having different colors. In the example illustrated, each ringhas a red segment, an or- 50 ange segment, a green segment, and a brownsegment, occurring in theorder named.

' The shield has a'keystone shaped opening below the arc-shaped opening,so that the colored segments of the dial vare visibleas the dial isturned.

. The shield 99 also has a number of arcs which Vcoincide with-the arcslion-'the dial 91, the shield arcsY Vhaving markedthereinithe:typenumbers of the standard vacuum-tubes which willbe testedduring the normal use of the meter. Y 5

In using the colored portion of the dial, the operator refers to theshield arcwhich is marked Withthe type number of the tube under test.'

The potentiometer contact member .8| is Vthen turned (the dial 91' beingturned with it). until it is set at the point where the reading of meter45Qisthe same Whetherthe testA switch 1| is open or closed; The operatorthen looks through the keystone opening at the colored segment of arcY70 on the dial 91 corresponding to the above-mentioned shield arc, andis informed at a glance whether` thetube is defective, questionable, orgood. If either the-redsegment Vor the-brown Ysegment is in front of theshield opening, the tube 75 isfdeiective; if the orange segment is infront 'I'he dial markingabove described is satisfactory Vof the opening,the tube is questionable; while if the green segment is in (front of theopening, the tube is good. It is only'in rare casesthat the brownsegment Willappear, showing thatthe mutualfcon'ductanceofa tubeis toohigh.Y

It will be understood that in practice a largel number of vacuum. tube f'sockets'would be provided for tubes to be tested,j although in'Fig. 1

only the wiring for vone such tube has beenV shown in order to simplifythe diagram. Preferably, these vacuum vtube sockets would be ymountedbelow and'in front of the vertical panel shown in Fig. 2. It' is alsoevident that-taps on the various secondary windings may `be provided tosupply the proper voltages to the various elements of any tube that maybe tested. It should be noted that the peak A. C. voltagefor theelements, except for the filament, is made equal to the desired D. C.potentials at'which the mutual conductance vof the tube isnormally'given by the manufacturer. Also, the proper peak grid volt-`age from winding 63 is made high by `one half Vofthe voltager across-R4so that the average bias is the rated' value. For filament type tubes anadditional correction for bias is needed because of peak instantaneousfilament voltages.

While the circuit illustrated applies voltages k tov grid and plate 180degrees out of phase, it may be desirable to apply the voltages in phasefor some tubes. lneither case, the peak potentialson grid and platevwill occur at the same instant.

' VFrom the'4 foregoing description itw'ill be apparent that even formeasuring plate current, the resistance values-ofthe potentiometers andresistance umts, except 81 and R3, are not critical.

In one embodiment of our invention, however,

the-following values were-found to be satisfactory: Potentiometer 79-400ohms; R4100 ohms;

VVlikt-650 ohms;'and vlts-4000 ohms. Y

InV the matter of measuring plate current, it

has been found that the potentiometer device 83--85 may be provided witha calibrated indicatingV means in-any suitablemanner, or, for example,an indicating means of the type provided in connection with thepotentiometer device 19-8I.

As has hereinbefore been pointed-out, the potentials derived from thepotentiometer'resistor 83'maybe'adjusted by movement of thepotentiometer contact 85 to provide a bucking current in theV winding 39to equal. the plate current,v or

to oppose the plate current to such a point that a desired zero-readingis obtained in the meter 45 in the circuit of the present example.

This is true since the current flowing through resistor 81 is'thecurrent that bucks out that portion of the plate current which wouldoverload the indicating device, so that this current is equal tothe'voltage determined by zthe percentage `of the potentiometer 83-85times the YVvoltage developed acrossthe winding 21. The Vcurrent is,then, this voltage divided by the resistance on the resistor'81. This,then, is equal to the plate current Y plus'thecurrent necessary to givethe zero read- Y ing on Ythe indicator '45.

A calibrated dial, or other indicating means for the potentiometercontact 85, may be provided, although it is not ordinarily included inan apparatus'of this character unless the additional cost is justified.However, it will be seen that, in any event, the position of thepotentiometer contact 85 may be taken Yas an approximation of the platecurrent.

A suitable' calibrated indicating'means is showndiagrammatically inFig.1-at MIZ-|03, in connection with the movable' contact 85. V In thisarrangement the pointer |02 is fixed, while the dial |03, rotatable withmovement of the contact 85, is calibrated in suitable units such asmilliamperes for a direct reading of plate current of the tube undertest.

The dial and pointer means, connected with the contact arm 8|, is alsoindicated diagrammatically in Fig. 1.

It has also been found that the apparatus shown may further besimplified and the cost thereby reduced without appreciably affectingthe accuracy or convenience of operation, by eliminating the volt meter13 with which the voltage adjusting device 15 is associated.

Normally, as hereinbefore pointed out, as a preliminary adjustmentbefore making a test, the input voltage to the test rapparatus isadjusted to a predetermined constant value by means of the potentiometerdevice 15, the reading being obtained from the volt meter 73. If it isdesired to omit the volt meter 13, or a similar separate voltageindicating device, the voltage may, however, be adjusted to a desiredconstant standard value by means of the adjusting device 15 by readingthe indicating meter 45, with the contact 85 on potentiometer 83 in theextreme left hand position, as viewed in the drawing, and with switch 1Iopen. With the tube to -be tested out of the socket, the potentiometerdevice 15 is then adjusted until the meter 45 reads a certainpredetermined value which has been found to correspond to a 'correctinput voltage for the test apparatus.

It will be seen that the current flowing in the plate circuit tube 3 isdependent upon the potentials applied to its'plate, filament, and grid.Since this grid potential is determined by the position of the switch 1Iand the potentiometer 85, it is necessary that these be in a zeroposition. The choice of these Zero reading positions ,of and 'H isarbitrary..v` However, as a matter of operating convenience, since thecontact 8l is also variable, the zero position for adjusting the inputvoltage is most easily and accurately obtained as above described, bymoving the contact 8.5 to the extreme Zero or left hand position and byopening the switch 1I to remove the Variable which would otherwise beintroduced into the reading by variations in the positions of thecontact 8l.

Various other modifications may be made in our invention withoutdeparting from the spirit and scope thereof, and'we desire, therefore,that only such limitations shall be placed thereon as are necessitatedby the'prior artvand set forth in the appended claims. l

We claim, as our invention:

l. In a mutual conductance meter for vacuum tubes, means for applyingalternating-current potentials to the grids and plates of said tubesapproximately degrees out o-f phase, means for changing thealternating-current potential on the grid whereby a changein platecurrent is produced, and means for utilizing said change in platecurrent to give'a direct reading of mutual conductance corresponding todirect-current potentials applied to the tube electrodes.

, 2. In a mutual conductance. meter for vacuum tubes, means for applyingalternating-current potentials to the grid and plate of the tube undertest so phased that the peak potentials occur at approximately the sameinstant, said potentials having a xed frequency, a circuit tuned to saidfrequency, an indicating device connected to said tuned circuit, 'meansfor impressing the output energy of said tube upon said tuned circuit,means for changing the potential on said grid whereby said output energyis changed, means for impressing upon said tuned circuit balancingenergy having said fixed frequency which is substantially 180 degreesout of Y phase with said tube output energy and which is equal to saidchan-gein tube output energy, and an indicating device associated withsaid last means.

3. A meter for determining the mutual conductance of a vacuum tube,rsaidmeter comprising means for supplying alternating-current voltages to thegrid and plate of said tube so phased that the peak potentials occur atapproximately the same instant, a circuit tuned to the frequency of saidvoltages, an indicating device connected to said tuned circuit, meansfor coupling the output circuit of said tube to said tuned circuitwhereby the energy in the plate for changing the grid potential of saidtube 'by a predetermined amount whereby a change'inplate current isproduced, and means for opposing said change in plate current, saidmeans including a low impedance potentiometer arin in series with arelatively high impedance device.

5. A meter for measuring the mutual conductance of a vacuum tubecomprising means for changing the grid potential of said tube by apredetermined amount whereby a change in plate current is produced,means for opposing said change in plate current, and a second means foropposing said change in plate current, said second means including a lowimpedance potenti'- ometer arm in series with a relatively highirnpedance unit. v

6. A meter 'for measuring the mutual conductance of a vacuumVV tubecomprisingY means for supplying voltages to the grid and plate of saidtube from an alternating current source, means forchanging the value ofthe voltage applied to said grid by a certain amount whereby a change inplate current is produced, means for indicating said change in platecurrent, and means including a potentiometer connectedto saidalternating current source whereby unrectied alternating current flowstherethrough for opposing said change in plate current. r

'7. A meter for measuring the mutual conductance of a vacuum tubecomp-rising Vmeans. for

supplying alternating-current voltagesgto the in series with saidbucking circuit.

8. A meter for measuring the Imutual con- Vductanc'eof a; vacuum tubecomprising meansV Vfor supplying alternating current voltages toV i thegrid-'and plate 'of said tube ina fixed phase relation, a `transformerhaving a primary winding in the plate. circuitgof said tube, thesecondary of Vsaid transformer-being connected toA an indicatingvdevice, meansffor changingthe grid potentialv of said tube a.v certainamount whereby achange in plate current is` produced, and meansYcomprising*k a variable source of potential connectedinuseriesfwith'said primary winding for opposing said V'change in platecurrent. ,A

Y 9..A meterfor measuring the4 mutualconductance of la. vacuum tubecomprising means frorapplying alternating voltages to the grid andplatevof the tube which areV out of lphase approximately ,180 degrees,means for indicating the resulting energy Ain-the plate-circuit of: saidtube,

means for changing ,the value 'of the alternating voltage applied to thegrid byf an amount equal toa constant times the voltage ofY analternating onet'er having a voltagemacross' it VWhichgis af constanttimes the voltage ofgsaidgsource, aportionY ofV the platelkgcircuit `of'the :tube being connectedin series a high impedanceuriit,'?sai1;iI n

portion and Ifsaid j high impedance unit` being Y Vconnected acrossafnarm ,ofosaidk potentiometer.,

Y 10. A meter for,determiningthe mutual Vcon-v ductance ofa'vacuumftubasaid metercomprisingVV meahsjfor'impressing. a'rxed amountof alternating voltagefro'mia source uponvthecontrol grid of 'saidtubeQa'plate. circuit for-said tube including `a source of alternatingcurrent and a transformerhavingI a, primary winding in series Ytherewith, the voltage of said source being applied t6 said plate out ofphasefwith the voltageV applied to said grid, saidtransformer havingasecondary Vwinding connectedtol 'an indicator and'tuned-to thefundamental frequency of saidsourceof altercurrent, a` source Iofalternating current of said fundamentali'frequency*shunted by a'Vpotentiometer, j means includinga portion ofV said potentiometerforchanging the grid voltage of said tube whereby the' platecurrentfthrough said Vtransformer primary may be changed, vand meanscomprising alportion Yof `said potentiometer' for impressing anadditional"potentialjacross,said

' transformer primary to bring the Venergy in-said A ,tuned Ysecondaryback to an vinitial value, said. G

potentiometerfbeingjlinearv in value'and being calibrated toV showmutual conductance directly.Y

1l; In a mutual conductance meter, means pro- A Vviding-aliode and gridcircuits for connection with. 'Y a vacuum `tubef device to be tested,Vmeans `for supplying to saidcircuits alternating current atvpeakpotentialssubstantially equalto the normal y operatingpotentials-forV said devicefand so phased that in the anodefand in oneof said grid'circuitsV the peak potentials occur .atfbthe' 'saine linstant;l an indicating devicei coupled ,to said anode'icircuit -forreceiving indications of changes in anodev cur-V rent therethrough;means for adjustingthe,V fiow of anodeY current in said circuit',` meansfor 'causing a predetermined change in theflowofanode current in saidcircuit from anadjustedyalue, variimpedance unit, 'said series'combination being:

able meansifor restoring said flow' tosaid rst-` namedfvalue, andanindicating dial'means connectedwith said last-named means for operationtherewith.Y

V12. In a mutual conductancemeter for testing a vacuum tube, means forapplying to the grid and Vplate of the tube alternating voltages of a bcertain frequency which are so phased that their f peak lpotentialsoccur at'the same instant, means for Yimpressing the resulting energy inthe plate 'circuitof said tube upon a circuit ltuned to said` certainfrequency, meansfor changing the value of the alternating voltageappliedtothe gridby a fixed amount wherebythe value of the plate ,circuitenergy is changed, and means for impress-V ing upon said tunedcircuitenergy which is in phase opposition to said plate circuit energy andofV` the same magnitude as said change in plate circuit energy, saidlast means including'a circuit having in series therewith apotentiometer arm which has low impedancecompared with the im- Vpedanceof said last-named circuit.

13. In a mutual conductance meter for testing Y a vacuum tube, vmeans,for applying `to the grid and plateofgthe tubealternating voltages of aV `certain frequency'which are sophased that their peak potentials occurat the same instant, means. f for impressing thejresulting energy` inthe plate circuit of said tubeupona circuit tuned to said certainfrequency, means for changing the value of thealternating;voltageapplied to the igridby i ,a 1fixed, amount whereby z the `valueVof the .plate circuit energy is changed, and means-for lmpress.-

ing upon said tuned circuit lenergy which 'isdn phase opposition to saidplate circuit Venergy and ofthe same magnitude assaidchange in platecircuit energy, said last means including aV circuit 'having inseriestherewith a potentiometer arm which has low impedance comparedwith the imimpedance of said last-named circuit, said potentiometer armAbeing circular in form and having a. mutual conductance indicating dialin the form l of a, circular plate connected thereto for adjust'-menttherewlth.

l Y 14. In a mutualconductancje meter, a common 5 Vsourceof alternatingcurrent including a supply transformer-having :windings which when ener-'giz'ed'provi'de 'peakealternating potentials substan-` tially equal tothe normal operating potentials `required for' avacuum. tube to betested. acathode vsl'ipplyleadjan anode'.'gridfand anodeV circuitsconnected'with said windingsgto receive-said potentials in substantiallyoppositephase relation to Y each-.other, ya vacuumtube vvoltmeter, acoupling fdevice therefor in said anode circ'uit,'potentiom eter'meansconnected with-one of said windings and Vat one terminal thereof withone terminal of the coupling device and the cathode lead, a switch incircuit between the opposite terminal and the last-named winding,"'a tapon" said potentiometer A.means for the'gridlcircuit, a second tap onsaid potentiometer meanslconnectedwlth the opposite terminal 'of thecoupling device, and a calibrated dial means connected with said-tap tomove there- Y 715. The-method of' employing apparatus lnl' with.

cluding a tuned circuit for'deterininngr the mutual Y conductance of'arvacuum tube,";said circuit being" tuned tofa certain frequency, whichVmethod 'comprises applying to the grid and plate ofthe tubevalternating]voltages of said certain'frequency which aresophasedthatltheir peak potentials said -tunedclrcuit, changing-'ftheVvalue-` o! time` VAoccur atthesame instant, impressing'the result- I Ying-energy in the plate circuit ofsaid tube upon alternating voltageapplied to the grid by a fixed amount whereby the value of the platecircuit energy is changed, impressing upon said tuned circuit otherenergy which is in phase opposition to said plate circuit energy and ofthe same magnitude as said change in plate circuit energy, and producingan indication proportional to said other energy which indication is ameasure of said mutual conductance.

16. In combination, a socket for a vacuum tube to be tested, means forsupplying to the terminals of said socket alternating voltages havingpeak values equal to the rated direct-current voltages for said tube,and means for obtaining a direct reading of the mutual conductance ofsaid tubex said reading having an accuracy approximately the same as theaccuracy of a measurement made with said rated direct-current voltagesapplied to said tube.

17. In a testing device for electric discharge devices, means includingtransformer windings providing testing potentials for application to theelectrodes thereof in a predetermined phase relation in accordance witha normal operating condition, an electrical indicating device includinga meter, means providing an anode circuit for said electric dischargedevice to be tested, a coupling device between said anode circuit andsaid electrical indicating device, a potentiometer device having animpedance element connected with an alternating current supply windingand having a movable contact connected with said anode ciru cuit wherebyan alternating current in opposition to a normal anode current may beapplied there to, and the impedance of said potentiometer device beingso related to the impedance of the anode circuit that an indication ofthe anode current is established by the adjusted position of thepotentiometer contact when a predetermined reading is obtained on themeter in the input indicating device.

LOY E. BARTON.

CARL C. CHAMBERS.

